1. Field of the Invention
The present description relates to a method and system for non-invasively measuring the concentration of a substance, in particular, the concentration of glucose in a biological entity.
2. Background
In certain instances, it is necessary to measure the concentration of particular substances in a biological entity, such as a biological entity as a human. Commonly used test procedures for measuring such concentrations are invasive, requiring the drawing of blood. This can be particularly unpleasant for individuals who need to obtain concentration measurements at frequent intervals. For example, diabetic patients need to monitor the levels of glucose in their bloodstream and are required to undergo such invasive measurement procedures on a daily basis, often several times a day. Typically, the measuring is done through a finger prick to draw blood, which is placed on a test strip that is then inserted into a glucose monitoring device.
To avoid the discomfort and inconvenience of invasive testing, non-invasive methods of measuring the concentration of blood stream components of interest, such as glucose, have been developed. It is well known that the properties of glucose can rotate polarized light. In addition, changes of glucose concentration within a solution can affect the absorption of light and change the refractive index of light as it transfers to and from other optical mediums.
One location that is suitable for performing non-invasive glucose measurements is the aqueous humor of the eye. Another is the vitreous humor of the eye. The concentration of glucose in the aqueous and vitreous humor directly relate to the concentration of glucose in the bloodstream. However, the relationship between the concentration of glucose in eye fluids and its effects on light transmitted through the eye fluids is difficult to use for purposes of determining the concentration of glucose in the bloodstream. In part, this difficulty stems from the fact that accurately measuring the optical path length (l) is difficult in a structure having a geometry as complex as that of an eye. In addition, known techniques for measuring the rotation angle of plane polarized light and very small changes in the refractive index are difficult to reliably implement outside of a laboratory setting; particularly, the measurements are difficult to obtain in a setting in which such non-invasive testing will be performed by the patient or by a technician. As a result, a need has developed for a method and system that address the foregoing problems.